An attenuator requires a high input impedance to minimize loading of the input signal while providing typical switchable signal attenuation ratios of 1:1 (1.times.), 10:1 (10.times.), and 100:1 (100.times.). The electrical characteristics of such an attenuator must be stable and accurate when subjected to input signals having bandwidths ranging from DC to 1 GHz and voltages ranging up to 500 peak volts. The attenuator must also have a low voltage-standing-wave-ratio (VSWR) and impart few aberrations to the input signal as it passes through the attenuator. Moreover, the entire assembly must be well shielded to prevent the coupling of undesired external signals into the attenuator circuits.
U.S. Pat. No. 4,495,458 of Murphy et al., assigned to the assignee of the present patent application, describes attenuator circuitry meeting many of the above requirements. FIG. 1 is a simplified schematic diagram of a prior art attenuator similar to that described by Murphy et al. The attenuator includes divider circuits 10 and 12 that are selectively connected in electrical series by divider relays 14, 16, 18, and 20. Input signals applied to an input 22 are coupled to divider circuits 10 and 12 through a coupling relay 24 that selects AC or DC coupling. A termination relay 26 connects a 50 ohm termination resistor 28 to the input signal when desired. A buffer amplifier 30 provides the necessary high-input impedance for minimal loading of the input signal and has a low-output impedance necessary for driving measurement circuits. FIG. 1 also shows a set of probe coding contacts 32 that surround input 22. Such contacts are typically used to detect particular characteristics of measurement probes such as attenuation ratio and to provide power and offset voltage signals to probes having active electrical circuits.
The manufacture of an attenuator of the type shown in FIG. 1 is costly and typically requires numerous miniature components mounted on subassemblies, thereby requiring tedious assembly with numerous interconnections and fasteners. Post-assembly calibration of divider stages is necessary and is subject to electrical interaction between stages. Such an attenuator assembly is especially disadvantageous for use in oscilloscopes because they typically require four attenuators, which are typically manufactured one-at-a-time.
U.S. Pat. No. 5.032,801 for a "High Performance Attenuator Configuration" to Woo et al., assigned to the assignee of the present patent application, describes an attenuator having multiple parallel voltage divider paths that are switched by a combination of relays and active circuits. The attenuator topology of Woo et al. requires fewer, less costly relays and has shorter overall circuit path lengths that result in lower VSWR and aberrations than those present in conventional attenuators. The parallel path topology also eliminates electrical interaction between dividers during calibration. However, the assembly, interconnection, and shielding, of the attenuator is not addressed in the application.
UK Pat. Application No. GB 2 155 251A describes the use of a conductive elastomer for the interconnection of integrated circuits to substrates, ribbon cables to circuit boards, and other typical interconnection schemes. A useful property of conductive elastomers is that they contain a plurality of minute conductors that conduct linearly through the material without conducting laterally within the material. Hewlett-Packard Corporation, Palo Alto, Calif., manufactures the 54500 series of oscilloscopes that are known to use conductive elastomers for ground returns in attenuator circuits. However, the Hewlett-Packard attenuators are assembled one-at-a-time, and use cascade path topology requiring expensive relays.